UAS Chromatograph for Atmospheric Trace Species (UCATS)

The 2-channel UCATS GC utilizes rapid gas chromatography to measure five trace gases every 70 sec.: nitrous oxide, N₂O; sulfur hexafluoride, SF₆; chlorofluorocarbon (CFC)-11, CCl₃F; CFC-12, CCl₂F₂; and halon-1211, CBrClF₂. Each channel is comprised of a 12-port valve for sample injection, a 1.5 cm³ sample loop, narrow bore (2.2 mm id) packed columns, and an electron capture detector. The GC carrier gas and calibration gas are contained in aircraft certified high-pressure cylinders mounted external to the instrument box. A Teflon diaphragm pump, also external to the GC, flushes air samples through the GC for their subsequent analyses. The N₂O/SF₆ measurements require doping of one ECD with a miniscule flow of carbon dioxide from a small, high-pressure cylinder inside the instrument. For extended low-tropospheric flying, a small amount of desiccant (Mg(ClO₄)₂) is required to dry the sample air stream before it enters the GC. Calibration is performed in-flight every 5 min. through the analysis of a standardized cylinder of dried whole air. Further calibration is performed in the laboratory to characterize detector response functions over the full ranges of trace gas mixing ratios measured during flights. An on-board computer provides instrument control while continuously logging chromatographic and engineering data. Data are recorded on removable media (PCMCIA flash disk) during flight, then are archived off-line after landing. In the post-flight phase, chromatograms are processed into mixing ratio data with error estimates for each measurement.

• Accuracy: ± 1%
• Precision: ± 0.2-0.5% (N₂O, CFC-11, CFC-12), ± 0.8-1.0% (SF₆), ± 1.5-3.0% (halon-1211)
• Weight: 80 lbs.
• Power: 28VDC, 100W typical, 300W maximum
• Response Time: 70 sec.

The OZ photometer is a small, low-power instrument that measures ozone (O₃) mixing ratios in the air stream every 10 sec. The instrument, which resides inside the UCATS GC box, is nearly self-contained, having it’s own air pump, instrument control software, and data acquisition board, needing only its power from the GC power distribution system. The photometer requires its own Teflon inlet tubing that is void of any wetted metal surfaces. The ozone measurements are based on the absorption of UV light (254 nm) by the ozone present in air as it flows through a 15-cm long absorption cell at approximately 1 L min^-1. The intensities of light reaching the photodiode detector after alternately passing through ozone-free air (IO) and sampled air (I) in the cell are used in a simple Beer-Lambert Law calculation of the ozone mixing ratio. Photodiode signals and housekeeping data such as cell pressure and temperature are recorded on a PCMCIA compact flash card which is removed post-flight for data processing. The ozone data are also logged by the GC data acquisition system. The photometer is calibrated in the laboratory before and after flights.

• Accuracy: ±5 ppb
• Precision: ±3 ppb
• Weight: included in GC weight
• Power: included in GC power
• Response Time: 10 sec.